U.S. patent number 9,048,060 [Application Number 14/056,710] was granted by the patent office on 2015-06-02 for beam pulsing device for use in charged-particle microscopy.
This patent grant is currently assigned to FEI COMPANY. The grantee listed for this patent is FEI Company. Invention is credited to Alexander Henstra, Erik Rene Kieft, Fredericus Bernardus Kiewiet, Adam Christopher Lassise, Otger Jan Luiten, Petrus Henricus Antonius Mutsaers, Edgar Jan Dirk Vredenbregt.
United States Patent |
9,048,060 |
Kieft , et al. |
June 2, 2015 |
Beam pulsing device for use in charged-particle microscopy
Abstract
The invention relates to a charged-particle microscope
comprising a charged-particle source; a sample holder; a
charged-particle lens system; a detector; and a beam pulsing
device, for causing the beam to repeatedly switch on and off so as
to produce a pulsed beam. The beam pulsing device comprises a
unitary resonant cavity disposed about a particle-optical axis and
has an entrance aperture and an exit aperture for the beam. The
resonant cavity is configured to simultaneously produce a first
oscillatory deflection of the beam at a first frequency in a first
direction and a second oscillatory deflection of the beam at a
second, different frequency in a second, different direction. The
resonant cavity may have an elongated (e.g. rectangular or
elliptical) cross-section, with a long axis parallel to said first
direction and a short axis parallel to said second direction.
Inventors: |
Kieft; Erik Rene (Eindhoven,
NL), Kiewiet; Fredericus Bernardus (Eindhoven,
NL), Lassise; Adam Christopher (Utrecht,
NL), Luiten; Otger Jan (Eindhoven, NL),
Mutsaers; Petrus Henricus Antonius (Geldrop, NL),
Vredenbregt; Edgar Jan Dirk (Eindhoven, NL), Henstra;
Alexander (Utrecht, NL) |
Applicant: |
Name |
City |
State |
Country |
Type |
FEI Company |
Hillsboro |
OR |
US |
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Assignee: |
FEI COMPANY (Hillsboro,
OR)
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Family
ID: |
47296933 |
Appl.
No.: |
14/056,710 |
Filed: |
October 17, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140103225 A1 |
Apr 17, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61714822 |
Oct 17, 2012 |
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Foreign Application Priority Data
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Oct 22, 2012 [EP] |
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12189369 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
37/045 (20130101); H01J 37/28 (20130101); H01J
2237/0432 (20130101) |
Current International
Class: |
H01J
37/26 (20060101) |
Field of
Search: |
;250/305,306,307,309,310,311,492.1,492.3,396R,397 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Fehr, J., et al., "A 100-Femtosecond Electron Beam Blanking
System," Microelectonic Engineering, 1990, pp. 221-226, vol. 12.
cited by applicant .
Lassise, A., et al., "Compact, low power radio frequency cavity for
femtosecond electron microscopy," Review of Scientific Instruments,
2012, 10 pages, vol. 83. cited by applicant.
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Primary Examiner: Ippolito; Nicole
Attorney, Agent or Firm: Scheinberg & Associates, PC
Scheinberg; Michael O. Hillert; John E.
Parent Case Text
This application claims priority from U.S. Provisional Application
61/714,822, filed Oct. 17, 2012, which is hereby incorporated by
reference.
Claims
The invention claimed is:
1. A charged-particle microscope comprising: a charged-particle
source, for producing a beam of charged particles that propagates
along a particle-optical axis; a sample holder, for holding and
positioning a sample; a charged-particle lens system, for directing
said beam onto the sample held by the sample holder; a detector,
for detecting radiation emanating from the sample as a result of
its interaction with the beam; a beam pulsing device, for causing
the beam to repeatedly switch on and off so as to produce a pulsed
beam; and an oscillating power supply connected to the beam pulsing
device, wherein the beam pulsing device comprises a unitary
resonant cavity disposed about said particle-optical axis and
having an entrance aperture and an exit aperture for the beam, the
unitary resonant cavity being configured to independently adjust a
pulse length and a pulse frequency of the pulsed beam by:
simultaneously producing a first oscillatory deflection of the beam
at a first frequency in a first direction and a second oscillatory
deflection of the beam at a second, different frequency in a
second, different direction, altering a frequency difference
between the first frequency and the second frequency to adjust the
pulse frequency; and adjusting an amplitude of an output of the
oscillating power supply to the unitary resonant cavity to adjust
the pulse length.
2. The charged-particle microscope according to claim 1, wherein
said unitary resonant cavity: is substantially cylindrical in form,
with a cylindrical axis that is substantially collinear with said
particle-optical axis; and is configured to be excited in
TM.sub.110 resonant mode.
3. The charged-particle microscope according to claim 1, wherein,
when viewed in a direction normal to said particle-optical axis,
the unitary resonant cavity has an elongated cross-section, with a
long axis parallel to said first direction and a short axis
parallel to said second direction.
4. The charged-particle microscope according to claim 3, wherein
said cross-section is substantially an ellipse whose major and a
minor axis correspond respectively to said first and second
directions.
5. The charged-particle microscope according to claim 3, wherein
the unitary resonant cavity is mechanically deformable so as to
adjust a ratio of the lengths of said long and short axes.
6. The charged-particle microscope according to claim 1, wherein
said unitary resonant cavity comprises a dielectric material that
is disposed about said particle-optical axis and that contains a
substantially axially symmetric void through which the beam can
pass.
7. The charged-particle microscope according to claim 6, wherein an
interface between said void and said dielectric material is at
least partially coated by a film of electrically conductive
material.
8. The charged-particle microscope according to claim 1, wherein:
in operation, said unitary resonant cavity is configured to cause
the beam to trace out a composite geometrical figure on a masking
plane perpendicular to the particle-optical axis; and the
microscope comprises a masking plate located in said masking plane
and having an opening that can be positioned so as to intersect
said composite geometrical figure, thus serving to admit a pulse of
charged particles as the beam traces across said opening.
9. The charged-particle microscope according to claim 8, wherein
said composite geometrical figure is a Lissajous figure.
10. The charged-particle microscope according to claim 1,
additionally comprising apparatus for applying a stimulus to the
sample, which stimulus can be synchronized to the output of the
beam pulsing device.
11. A method of examining a sample using a charged-particle
microscope, comprising: providing the sample on a sample holder;
using a charged-particle source to produce a beam of charged
particles that propagates along a particle-optical axis; using a
beam pulsing device to repeatedly switch the beam, thus producing a
pulsed beam; using a charged-particle lens system to direct the
pulsed beam onto the sample; using a detector to detect radiation
emanating from the sample as a result of its interaction with the
beam, employing a unitary resonant cavity as part of the beam
pulsing device, disposing this cavity about the particle-optical
axis, and passing the beam through the cavity via entrance and exit
apertures; exciting the unitary resonant cavity to simultaneously
produce a first oscillatory deflection of the beam at a first
frequency in a first direction and a second oscillatory deflection
of the beam at a second, different frequency in a second, different
direction; adjusting a pulse frequency of the pulsed beam by
altering a frequency difference between the first frequency and the
second frequency; and adjusting a pulse length of the pulsed beam
by adjusting an amplitude of an output of an oscillating power
supply connected to the unitary resonant cavity.
12. A beam pulsing device for use with a charged-particle
microscope, the beam pulsing device causing a charged-particle beam
to repeatedly switch on and off so as to produce a pulsed beam, the
beam pulsing device comprising: a unitary resonant cavity disposed
about a particle-optical axis of the charged-particle microscope
and having an entrance aperture and an exit aperture for the
charged particle beam, the unitary resonant cavity being configured
to independently adjust a pulse length and a pulse frequency of the
pulsed beam by: simultaneously producing a first oscillatory
deflection of the beam at a first frequency in a first direction
and a second oscillatory deflection of the beam at a second,
different frequency in a second, different direction, altering a
frequency difference between the first frequency and the second
frequency to adjust the pulse frequency; and adjusting an amplitude
of an output of an oscillating power supply connected to the
unitary resonant cavity to adjust the pulse length.
13. The beam pulsing device of claim 12, wherein said unitary
resonant cavity: is substantially cylindrical in form, with a
cylindrical axis that is substantially collinear with said
particle-optical axis; and is configured to be excited in
TM.sub.110 resonant mode.
14. The beam pulsing device of claim 12, wherein, when viewed in a
direction normal to said particle-optical axis, the unitary
resonant cavity has an elongated cross-section, with a long axis
parallel to said first direction and a short axis parallel to said
second direction.
15. The beam pulsing device of claim 14, wherein said cross-section
is substantially an ellipse whose major and minor axis correspond
respectively to said first and second directions.
16. The beam pulsing device of claim 14, wherein the unitary
resonant cavity is mechanically deformable so as to adjust a ratio
of the lengths of said long and short axes.
17. The beam pulsing device of claim 12 wherein said unitary
resonant cavity comprises a dielectric material that is disposed
about said particle-optical axis and that contains a substantially
axially symmetric void through which the beam can pass.
18. The beam pulsing device of claim 17, wherein an interface
between said void and said dielectric material is at least
partially coated by a film of electrically conductive material.
19. The beam pulsing device of claim 12, wherein: in operation,
said unitary resonant cavity is configured to cause the beam to
trace out a composite geometrical figure on a masking plane
perpendicular to the particle-optical axis; and the
charged-particle microscope comprises a masking plate located in
said masking plane and having an opening that can be positioned so
as to intersect said composite geometrical figure, thus serving to
admit a pulse of charged particles as the beam traces across said
opening.
20. The beam pulsing device of claim 19, wherein said composite
geometrical figure is a Lissajous figure.
Description
The invention relates to a charged-particle microscope comprising:
A charged-particle source, for producing a beam of charged
particles that propagates along a particle-optical axis; A sample
holder, for holding and positioning a sample; A charged-particle
lens system, for directing said beam onto a sample held on the
sample holder; A detector, for detecting radiation emanating from
the sample as a result of its interaction with the beam; A beam
pulsing device, for causing the beam to repeatedly switch on and
off so as to produce a pulsed beam.
The invention also relates to a method of using such a
charged-particle microscope.
As used throughout this text, the ensuing terms should be
interpreted as follows: The term "charged particle" encompasses an
electron or ion (generally a positive ion, such as a Gallium ion or
Helium ion, for example, though a negative ion is also possible).
It may also be a proton, for example. The term "charged-particle
microscope" (CPM) refers to an apparatus that uses a
charged-particle beam to create a magnified image of an object,
feature or component that is generally too small to be seen in
satisfactory detail with the naked human eye. In addition to having
an imaging functionality, such an apparatus may also have a
machining functionality; for example, it may be used to locally
modify a sample by removing material therefrom ("milling" or
"ablation") or adding material thereto ("deposition"). Said imaging
functionality and machining functionality may be provided by the
same type of charged particle, or may be provided by different
types of charged particle; for example, a Focused Ion Beam (FIB)
microscope may employ a (focused) ion beam for machining purposes
and an electron beam for imaging purposes (a so-called "dual beam"
microscope), or it may perform machining with a relatively
high-energy ion beam and perform imaging with a relatively
low-energy ion beam. The term "sample holder" refers to any type of
table, platform, arm, etc., upon which a sample can be mounted and
held in place. Generally, such a sample holder will be comprised in
a stage assembly, with which it can be accurately positioned in
several degrees of freedom, e.g. with the aid of electrical
actuators. The term "charged-particle lens system" refers to a
system of one or more electrostatic and/or magnetic lenses that can
be used to manipulate a charged-particle beam, serving to provide
it with a certain focus or deflection, for example, and/or to
mitigate one or more aberrations therein. In addition to (various
types of) conventional lens elements, the charged-particle lens
system (particle-optical column) may also comprise elements such as
deflectors, stigmators, multipoles, aperture (pupil) plates, etc.
The phrase "radiation emanating from the sample" is intended to
encompass any radiation that emanates from the sample as a result
of its irradiation by the charged-particle beam. Such radiation may
be particulate and/or photonic in nature. Examples include
secondary electrons, backscattered electrons, X-rays, visible
fluorescence light, and combinations of these. Said radiation may
also simply be a portion of the incoming beam that is transmitted
through or reflected from the sample, or it may be produced by
effects such as scattering or ionization, for example. The term
"detector" should be broadly interpreted as encompassing any
detection set-up used to register (one or more types of) radiation
emanating from the sample. Such a detector may be unitary, or it
may be compound in nature and comprise a plurality of
sub-detectors, e.g. as in the case of a spatial distribution of
detector units about a sample holder, or a pixelated detector. The
detector may be used in image formation and/or for spectroscopic
investigation (e.g. as in the case of techniques such as EDX or WDX
(Energy- or Wavelength-Dispersive X-ray Spectroscopy), EELS
(Electron Energy-Loss Spectroscopy) and EFTEM (Energy-Filtered
Transmission Electron Microscopy)).
In what follows, the invention will--by way of example--often be
set forth in the specific context of electron microscopes. However,
such simplification is intended solely for clarity/illustrative
purposes, and should not be interpreted as limiting.
Electron microscopy is a well-known technique for imaging
microscopic objects. The basic genus of electron microscope has
undergone evolution into a number of well-known apparatus species,
such as the Transmission Electron Microscope (TEM), Scanning
Electron Microscope (SEM), and Scanning Transmission Electron
Microscope (STEM), and also into various sub-species, such as
so-called "dual-beam" tools (e.g. a FIB-SEM), which additionally
employ a "machining" beam of ions, allowing supportive activities
such as ion-beam milling or ion-beam-induced deposition, for
example.
In traditional electron microscopes, the imaging beam is
continuously "on" during a given imaging session. However, in
recent years, the possibility of being able to perform
"time-resolved" microscopy using a pulsed electron beam has
attracted interest. In such microscopy, the use of a pulsed input
beam allows output from the employed detector to be discretized
into a temporal train of timestamped components (e.g. images or
spectra). The principle behind such microscopy can be compared to
the principle underlying stroboscopic photography, where the use of
a high-speed flash allows continuous motion of a photographed
object to be captured as a temporal train of freeze-frame
exposures--facilitating accurate analysis of subtle differences
between the captured frames. By using a pulsed (or "flashed")
charged-particle beam in a CPM, it becomes possible to accurately
investigate dynamic processes in a sample, such as phase
transitions, mechanical vibrations, heat dissipation, chemical
reactions, biological cell division, fluid flow, electrical
response, radioactive decay processes, etc.
Of particular interest as regards the technique elucidated in the
previous paragraph is the ability to produce ultra-short-duration
charged-particle pulses, since these can in turn be used to
investigate ultra-fast dynamic processes. One known way of
producing such pulses is to embody the charged-particle source to
comprise a photo-electric emitter (e.g. a heated LaB.sub.6
crystal), and embody the beam pulsing device as a pulsed laser beam
that irradiates said emitter with ultra-short bursts of light (e.g.
with a duration of the order of picoseconds (ps) or femtoseconds
(fs)); such a technique is set forth, for example, in US
2005/0253069 A1. Since it is relatively easy to acquire and use
lasers that are capable of producing extremely short light pulses,
this particular method would appear to lend itself to ultra-fast
time-resolved analysis of samples. However, a significant drawback
of this known technique is that the employed photo-electric emitter
produces a much lower electron brightness than is typically
available from a conventional (e.g. Schottky) source in an electron
microscope, which severely limits the practical usefulness of this
approach.
In an alternative solution, one could elect to use a conventional
electron source (such as the aforementioned Schottky source), and
embody the beam pulsing device as a beam "chopper" or "blanker"
that interrupts/passes the electron beam in an oscillatory manner.
For example, the beam pulsing device might employ a beam deflector
in which electrodes generate an oscillatory electric field that
periodically laterally deflects the electron beam away from a
nominal particle-optical axis, thus effectively creating a pulsed
beam further downstream. In a more sophisticated variant, one could
use a resonant cavity to produce said beam deflection. Although
this method has the advantage of using a high-brightness electron
source, it is generally of limited flexibility as regards its
application to practical beam chopping. In particular, this method
does not lend itself to the production of short pulses (e.g. with a
pulse lengths in the fs-ps range) at relatively low frequencies
(e.g. of the order of 100 MHz), since the former aspect (short
pulses) requires a relatively high deflection frequency whereas the
latter aspect (relatively long period between pulses) requires a
relatively low deflection frequency, and these two different
demands are difficult to mutually reconcile. For an example of this
approach, reference is made to the article by K. Ura et al.,
"Picosecond Pulse Stroboscopic Scanning Electron Microscope", J.
Electron Microsc., Vol. 27, No. 4 (1978), pp. 247-252.
In a variant of the approach set forth in the previous paragraph,
one could attempt to embody the beam pulsing device as a series
configuration of two "crossed" deflectors with intermediate drift
space; here, the term "crossed" is used to indicate that the
deflection direction (e.g. along an x-axis) of one deflector is
perpendicular to the deflection direction (e.g. along a y-axis) of
the other deflector. The idea here is that the input to the second
deflector is pulsed by the first deflector, so that the second
deflector produces "a pulse of a pulse" or, in effect, a beat. Such
a configuration would allow more flexibility, in that there are now
different frequencies that can be adjusted (deflection frequencies
of first and second deflections, and beat frequency of the
superimposed deflections) so as to allow more independent variation
of the pulse length and pulse frequency of the output (resultant)
pulse. However, it should be noted that use of such a set-up in a
CPM beam pulsing device would tend to significantly complicate the
employed particle-optical column between source and sample. This is
because, in order to work satisfactorily, each deflector is ideally
situated at the focal point of a lens (located upstream in the
particle-optical column), e.g. a condenser/objective lens. If one
employs an x-deflector at position z.sub.1 along the
particle-optical axis and a y-deflector at position z.sub.2 along
the same axis, then a first stigmator will have to be used upstream
of the deflectors so as to deliberately introduce enough
astigmatism to give each deflector its respective focus, and a
second stigmator will have to be used downstream of the deflectors
in order to subsequently mitigate this deliberately introduced
astigmatism. Because the two deflectors are in series arrangement
and have an intermediate drift space, the distance
.DELTA.z=|z.sub.2-z.sub.1| will be relatively large (e.g. of the
order of a few cm), thus requiring relatively large and powerful
stigmators--which tends to be a significant disadvantage in a
(typically) cramped particle-optical column.
It is an object of the invention to address these issues. More
specifically, it is an object of the invention to provide a CPM in
which ultra-fast time-resolved microscopy can be satisfactorily
performed. In particular, it is an object of the invention that
such a CPM should employ a charged-particle beam pulsing device
with which it relatively easy to (independently) adjust the
obtained pulse length and pulse frequency, without introducing
excessive astigmatism. More specifically, it is an object of the
invention that such a beam pulsing device be capable of producing
ultra-short beam pulses (e.g. with ps or fs pulse lengths) at
relatively low frequencies (e.g. of the order of 100 MHz).
Moreover, it is an object of the invention that the charged
particle beam in such a CPM should have satisfactory
brightness.
These and other objects are achieved in a charged-particle
microscope as set forth in the opening paragraph, characterized in
that the beam pulsing device comprises a unitary resonant cavity
disposed about said particle-optical axis and having an entrance
aperture and an exit aperture for the beam, which resonant cavity
is embodied to simultaneously produce a first oscillatory
deflection of the beam at a first frequency in a first direction
and a second oscillatory deflection of the beam at a second,
different frequency in a second, different direction.
The beam pulsing device in the CPM according to the present
invention simultaneously produces two different deflections in a
single (unitary) resonant cavity; consequently, there is no longer
a need to use the powerful stigmators referred to above, because
the abovementioned substantial focus separation .DELTA.z is no
longer present. The absence of such large stigmators, coupled with
the much more compact size of the inventive beam pulsing device
parallel to the particle-optical axis direction (single cavity
rather than dual cavities with intermediate drift space), results
in very significantly reduced demands on available space in the
particle-optical column of the CPM. The beam pulsing device
according to the present invention is thus more space-saving and
produces fewer aberration issues.
In a particular embodiment of the invention, said unitary resonant
cavity is: Substantially cylindrical in form, with a cylindrical
axis that is substantially collinear with said particle-optical
axis (z-axis); Embodied to be excited in TM.sub.110 resonant
mode.
It should be noted that the term "cylindrical" is used here in a
strict mathematical sense, and thus encompasses cylinders that do
not have a circular cross-section. According to standard usage in
the field of electromagnetism, the symbol "TM" indicates a
Transverse Magnetic field, i.e. an electromagnetic field that has
no longitudinal magnetic component (so that B=0 along the z-axis).
The triplet of subscripts "110" denotes integer eigenvalues of a
wave vector k needed to satisfy boundary conditions pertaining to
Maxwell's equations in the cavity. Without going into further
mathematical detail, a TM.sub.110 mode is a dipole mode with a
strong lateral magnetic field at radius r=0 (measured outward from
the z-axis) and zero electric field at r=0. Such a mode can, for
example, be excited in the cavity with the aid of a Hertzian dipole
loop antenna placed close to the wall of the cavity (distal from
the z-axis). An antenna of this type can, for example, be achieved
by: Creating a small bore in a wall of the cavity; Feeding the
inner conductor of a coaxial cable through this bore to the
interior of the cavity, in such a way that said inner conductor
does not touch said (conducting) wall; Creating a loop in said
inner conductor proximal to said wall; Orienting the loop
appropriately (e.g. so that its plane is normal to the y-axis, to
excite a magnetic field parallel to y); Connecting said coaxial
cable to an oscillating Radio Frequency (RF) power supply.
The vibrational behavior of the cavity can be adjusted in various
ways. For example, the frequency of said oscillating power supply
can be altered. Alternatively, a small conducting (e.g. metallic)
or dielectric "plunger" (tuning element) can be partially inserted
into the cavity, e.g. through a small bore opposite the
above-mentioned antenna; the extent of insertion of such a plunger
will then influence the resonant frequency of the cavity, because:
Insertion of a conducting plunger will locally decrease the
effective radius of the cavity, with an attendant increase in
resonant frequency; Insertion of a dielectric plunger will increase
the effective dielectric constant of the cavity, with an attendant
decrease in resonant frequency.
Needless to say, when the cavity is excited on-resonance (i.e. the
frequency of the oscillating power supply is matched to the
resonant frequency of the cavity), the resulting electromagnetic
fields in the cavity will be at their largest. The skilled artisan
in the field of electromagnetism will be familiar with such
concepts, and will be able to implement and optimize them according
to the details/requirements of a particular configuration. In
particular, he will realize that other types and/or locations of
antenna (or other means of excitation) can be employed, as well as
other types and/or locations of tuning element/plunger. He will
also understand that he is not limited per se to a TM.sub.110
resonance mode, and that, in principle, other types of TM, TE
(Transverse Electric) and/or Transverse Electro-Magnetic modes may
be equally or better suited to a given set-up.
In order to simultaneously excite two different resonances of
mutually different frequency in the same cavity, one can, for
example, concurrently use two different excitement antennae (of a
type as described above, or similar), each antenna working in
unison with its own plunger/tuning element (again of a type as
described above, or similar). In such a set-up, one antenna/plunger
pair can be aligned so as to produce an oscillatory deflection
along said first direction, and the other plunger/antenna pair can
be aligned so as to produce an oscillatory deflection along said
second direction. The positions of the plungers and/or the driving
frequency of the antennae can then be adjusted to as to give said
oscillatory deflections the desired frequencies. However, there are
alternatives to such a set-up. For example, one could attempt to
use a single excitement antenna in conjunction with two different
plungers--though such an arrangement will generally be less
flexible than a two-antenna approach. Yet another alternative is
set forth in the next paragraph.
In a noteworthy embodiment of a CPM according to the current
invention, the resonant cavity--when viewed in a direction normal
to said particle-optical axis (z axis)--has an elongated
cross-section, with a long axis parallel to said first direction
and a short axis parallel to said second direction. In a particular
such embodiment, said long and short axes are substantially
perpendicular (though this is not strictly necessary). Because its
cross-section has two different characteristic dimensions, such a
resonant cavity can simultaneously support two different
resonances--one along each said dimension (as explained above,
resonant frequency depends inter alia on the (effective) internal
dimension of the cavity). In many practical applications of the
invention, only a relatively small frequency difference will be
required between said two resonances, so that the difference
between said two dimensions may be correspondingly small;
nevertheless, the current embodiment will generally allow larger
frequency differences to be achieved (if desired) than the set-up
described in the previous paragraph. Examples of cross-sectional
forms as alluded to here include rectangles and quasi-rectangular
forms such as "racetracks" (in which the two opposing straight
sides of a rectangle are replaced by curved sides).
In a particular example of an embodiment as set forth in the
previous paragraph, the resonant cavity's cross-section is
(substantially) an ellipse, with a major and a minor axis that
correspond respectively to said first and second directions. Such a
geometrical configuration is advantageous in that: An ellipse
doesn't contain any discontinuities in its form (e.g. corners,
angles), thus simplifying the electromagnetic field configurations
produced inside an elliptical cavity; An ellipse is a relatively
good approximation to a circular cross-section, with its various
symmetry-associated benefits. This is particularly the case for an
ellipse of relatively mild eccentricity; Even relatively mild
eccentricity of an ellipse provides enough scope to slightly vary
the effective internal dimensioning of the cavity in a particular
direction, for the purpose of resonant frequency adjustment.
In the current context, an oval or quasi-oval may be regarded as an
approximation to an ellipse.
To make an embodiment as set forth in either of the preceding two
paragraphs tunable, one could make use of tuning elements/plungers
as set forth above. However, as an alternative/supplement to such
an approach, one can also embody the resonant cavity to be
mechanically deformable so as to adjust a ratio of the lengths of
said long and short axes (major and minor axes). For example, one
could conceive a scenario whereby the cavity walls are (at least
partially) made of a pliable material (such as plastic) that is
coated with a film of metal or another conducting material; such
walls can then be locally nudged/squeezed/moved by appropriately
placed actuators so as to change their form/dimensioning/position,
thus (locally) altering the effective internal dimensions of the
cavity and, accordingly, its resonant behavior.
For good order, it should be noted that, in a cylindrical cavity
with a perfectly circular cross-section, and with perfectly smooth
walls that are uninterrupted by bores or protrusions, excitation of
a given resonance mode can, in fact, concurrently produce more than
one degenerate "versions" of the same mode; for example, one could
obtain two degenerate TM.sub.110 modes--one with a magnetic field
oriented along the x-axis and another with a magnetic field
oriented along the y-axis. However, the moment an imperfection is
introduced into such a cavity (e.g. by sliding in an antenna or
plunger, or by introducing a deformation of the circular
cross-section), such degeneracy is broken, and one of said
"versions" will become dominant. The embodiments in the preceding
paragraphs exploit this effect.
A resonant cavity as used in the current invention may, in a
relatively simple embodiment, contain a vacuum through which the
charged-particle beam propagates. However, in an alternative
embodiment, the inventive resonant cavity comprises a dielectric
material that is disposed about said particle-optical axis and that
contains a substantially axially symmetric void through which the
beam can pass. Use of a dielectric material in this manner allows a
higher field amplitude and frequency to be achieved for the same
input power, or, equivalently/alternatively, results in lower power
requirements to generate the same field amplitude and frequency (as
compared to a vacuum-filled cavity). Examples of candidate
dielectrics in this context include materials such as ZrTiO.sub.4,
sapphire, fused quartz, alumina, PTFE (PolyTetraFluoroEthylene) and
ZrO.sub.2, for instance. Said void is preferably substantially
axially symmetric with respect to the particle-optical axis--e.g.
(quasi-)conical or bell-shaped, or cylindrical--so as to allow the
charged-particle beam (in its various states of deflection and
non-deflection) to continue to traverse the cavity without
(significantly) intercepting the dielectric material.
In a refinement of an embodiment as set forth in the previous
paragraph, an interface between said void and said dielectric
material is at least partially coated by a film of electrically
conductive material. Put another way, the inward-facing surface of
the dielectric body that delimits said void is (at least partially)
metallized or coated with a (thin) film of other conductive
material, such as ITO (Indium Tin Oxide). In this way, the
accumulation of unwanted/parasitic electric charge at the interface
between the void and dielectric is advantageously mitigated--since
the presence of such charge would tend to disturb the
operation/deflection performance of the cavity. However, so as not
to disturb the intended electromagnetic field distribution inside
the cavity, the employed conductive film should be relatively thin.
More specifically, in order to make the film essentially
transparent to said field at the resonance frequency of the cavity,
the film thickness should be substantially smaller than the
so-called skin depth .delta. at that frequency, which is given by
the expression:
.delta..times..rho..omega..mu. ##EQU00001##
where .rho. and .mu. are, respectively, the resistivity and
magnetic permeability of the film material, and .omega.=2.pi.f is
the angular frequency corresponding to a linear frequency f. As an
example, for copper (.rho.=1.71.times.10.sup.-8 .OMEGA.m,
.mu..apprxeq..mu..sub.3=4.pi..times.10.sup.-7NA.sup.-2) at a
resonance frequency of 3 GHz, .delta.=1.2 .mu.m. For (non-magnetic)
film materials with higher resistivity, the skin depth will be
larger. Needless to say, if the film is relatively thin, then its
DC conductance will be accordingly low; however, even a relatively
poor conductor at the surface of the dielectric will be capable of
preventing a gradual build-up of electric charge.
The discussion above has been largely structural in nature, but it
is also possible to give a more functional description of the
operation of the invention. In this respect, it should be
remembered that: When two waveforms of (slightly) different
frequency are superimposed, one obtains a train of beats, and the
frequency of these beats is (highly) sensitive to the frequency
difference between said superimposed waveforms. Similarly, the
frequency of the beam pulses observed at sample level in a CPM
according to the present invention can be tuned by altering the
frequency difference between the two resonances that are
simultaneously produced in the unitary resonant cavity of the
inventive beam pulsing device. If the frequency of a deflection is
kept constant and its amplitude is increased, then the linear speed
of the deflection must also necessarily increase, and vice versa.
Accordingly, if one increases the amplitude of the output of the
oscillating power supply that is driving (at least one of the
resonances in) the inventive resonant cavity, the speed with which
the charged-particle beam is deflected back and forth will also
increase; a point situated on the deflection path of the beam will
thus experience a shorter pulse duration as the beam passes by. And
vice versa. By adjusting the relative phase between the two
oscillatory deflections in the inventive resonant cavity, one can
influence the location of a beat at a given moment in time. This
effect can, if desired, be exploited to influence spatial alignment
and/or synchronization aspects of the pulsed charged-particle beam,
as set forth in more detail below.
In further continuance of this discussion, an aspect of the current
invention is characterized in that: In operation, said resonant
cavity causes the charged-particle beam to trace out a composite
geometrical figure (e.g. a so-called Lissajous figure) on a masking
plane perpendicular to the particle-optical axis; The microscope
comprises a masking plate located in said masking plane and having
an opening that can be positioned so as to intersect said composite
geometrical figure, thus serving to admit a pulse of charged
particles as the beam traces across said opening.
As set forth above, a resonant cavity as specified by the current
invention is capable of simultaneously exciting two different
oscillatory deflections (of a charged-particle beam) in two
different lateral directions and at mutually different frequencies.
The net effect/resultant of these two deflections will be to cause
the beam to "dance" out a composite (and periodic) geometrical
figure on a plane normal to the (nominal) particle-optical axis.
More specifically, in the case of sinusoidal oscillations along the
x- and y-axes (which may differ in amplitude, frequency and phase),
said figure will be a Lissajous figure--which term, in fact,
encompasses a whole family of figures whose shapes are sensitive to
the aforementioned amplitude, frequency and phase values. If, while
tracing out such a composite geometrical figure, the beam traverses
an opening (e.g. relatively small hole or slit) in a front side of
a masking plate, it will momentarily poke through the opening and
cause a beam pulse to be observed at the back side of the plate.
For a given geometrical figure, if said opening doesn't initially
lie at some point along the course of the figure, it can be made to
do so in various ways, e.g. by changing the position of the opening
(moving the plate or certain parts thereof) and/or adjusting the
aforementioned relative phase between the two oscillatory
deflections.
Even if the oscillating power supply drives the inventive cavity
with a non-sinusoidal waveform (such as a sawtooth or block wave),
it should be remembered that such waveforms can nevertheless be
decomposed into a sum of sinusoids (Fourier decomposition), whence
the discussion above remains pertinent.
For the sake of clarity, it should be explicitly remembered that
pulse length is not necessarily directly related to pulse
frequency, and that a pulse train may comprise extremely short
pulses (e.g. a few picoseconds or femtoseconds per pulse) at a
relatively "low" frequency (e.g. in the Megahertz range), simply
because there is a relatively long "dead time" between successive
pulses. The current invention allows independent adjustment of
pulse length and pulse frequency--an aspect that is inter alia of
importance in the context of the next paragraph.
The dynamic processes investigated using the CPM/method of the
current invention may, if desired, be deliberately
precipitated/maintained/steered by applying an appropriate external
stimulus (e.g. radiative, thermal, electrical, chemical, acoustic
and/or mechanical) to the sample under investigation, whereby the
application of such stimulus may, if desired/required, be matched
to the timing/phase of the pulsing behavior of the employed
charged-particle beam. For example, the sample under investigation
may have some property that can be influenced by the light and/or
heat that is locally delivered by a focused laser beam. The laser
in such an instance will typically deliver relatively short light
pulses that are separated by dead periods in which the lasing
cavity "reloads", e.g. resulting in picosecond pulses at a
frequency of, say, 75 MHz. It can be highly advantageous if a CPM
that is used to obtain imagery and/or spectroscopic information
from such a sample is capable of producing charged-particle beam
pulses that are matched (in terms of duration, frequency and phase)
to those of the laser. In this way, innovative analysis
techniques--such as high-brightness FEELS (Femtosecond Electron
Energy-Loss Spectroscopy)--can be exploited for sample analysis
purposes. It should be noted that the laser-based example given
here is not limiting. For example, one could apply an electrical
stimulus (e.g. inductively, or using a contact probe), or a
mechanical stimulus (using a vibrating membrane), etc.
The invention will now be elucidated in more detail on the basis of
exemplary embodiments and the accompanying schematic drawings, in
which:
FIG. 1A renders a transverse cross-section (in plan view) of a
unitary resonant cavity comprised in a beam pulsing device for use
in a charged particle microscope (CPM) according to the present
invention.
FIG. 1B shows a longitudinal cross-section (in elevation) of the
subject of FIG. 1A.
FIG. 1C shows magnetic and electrical field configurations in the
subject of FIGS. 1A and 1B for a TM.sub.110 resonance mode.
FIG. 1D illustrates a Lissajous figure that can be traced out by a
charged-particle beam traversing a resonant cavity as depicted in
FIGS. 1A and 1B, in operation.
FIG. 2 shows a longitudinal cross-sectional view of a particular
type of CPM (in this case, a TEM) in which the present invention
can be implemented.
FIG. 3 renders a longitudinal cross-sectional view of another type
of CPM (in this case, a SEM) in which the present invention can be
employed.
FIG. 4 depicts a longitudinal cross-sectional view of yet another
type of CPM (in this case, a FIB microscope) in which the present
invention can be put to use.
In the Figures, corresponding parts may be indicated using
corresponding reference symbols.
Embodiment 1
FIGS. 1A and 1B render various views of a unitary resonant cavity
201 comprised in a beam pulsing device for use in a charged
particle microscope (CPM) according to the present invention. More
particularly: FIG. 1A renders a lateral cross-sectional view of the
resonant cavity 201, observed along the z axis. When located in the
CPM, the cavity 201 will be placed/aligned so that the CPM's
particle-optical axis 219 extends along this z-axis; FIG. 1B
renders a longitudinal cross-sectional view of the same resonant
cavity 201, in which the z axis is now vertical.
Also illustrated are x and y axes, which form a Cartesian
coordinate system together with said z axis.
As depicted in FIG. 1A, the cavity 201 has a substantially
elliptical transverse cross-section, with a major axis parallel to
the x-axis (first direction) and a minor axis parallel to the
y-axis (second direction). The dashed circle 205 in FIG. 1A is
drawn centered on the cavity 201, to act as a reference to more
clearly reveal the elliptical form of the depicted cross-section.
As is evident from FIG. 1B, the cavity 201 is cylindrical in shape,
and its cylindrical axis is substantially collinear with the
depicted optical axis 219. Entrance aperture 221a and exit aperture
221b allow a charged-particle beam propagating along the optical
axis 219 to enter and leave the interior of the cavity 201,
respectively. A cavity of such form is sometimes referred to as a
"pillbox cavity".
In practice, entrance aperture 221a and exit aperture 221b may be
quite small, e.g. of the order of about a millimeter wide (whereby
it should be noted that a typical width of the charged-particle
beam will be of the order of about a few tens of microns); if
desired, exit aperture 221b may be somewhat wider, to allow for
greater deflection amplitudes of the charged-particle beam from the
particle-optical axis 219. In this respect, it should be noted that
apertures 221a and 221b are not drawn to scale.
Cavity wall 203 is made of conducting material, e.g. copper sheet
with a thickness of a few mm. Provided in this wall 203 at
successive angular intervals of 90.degree. about the z-axis and in
a common plane normal to the z-axis are small bores 207a, 207b,
207c, 207d, such that bores 207a, 207c oppose one another along the
y-axis and bores 207b, 207d oppose one another along the x-axis.
Through these bores 207a, 207b, 207c, 207d protrude respective rods
209a, 209b, 209c, 209d. Two of these rods--209a, 209b--carry
respective excitement antennae 211, 213, whereas the other two
rods--209c, 209d--carry respective tuning elements 215, 217. These
tuning elements 215, 217 take the form of plungers (disks) that can
be moved laterally into and out of the cavity 201, thus allowing
adjustment of the effective internal width of the cavity 201 along
the y- and x-axes, respectively (or serving to alter the net
dielectric constant of the cavity interior). The excitement
antennae 211, 213 are located proximal the wall 203, as far as
practicable from the particle-optical axis 219.
The excitement antennae 211, 213 are connected via coaxial cables
to respective (RF) Gigahertz oscillating power supplies (not
shown); in fact, as set forth above, one possible embodiment of the
antennae 211, 213 simply takes the form of a loop in the inner
conductor (core) of each such coaxial cable. The output of said
power supplies is adjusted so as to produce the desired
simultaneous resonances of the cavity 201--in the current example,
two TM.sub.110 resonance modes that are mutually perpendicular.
Because of the elliptical cross-section of the cavity 201, with the
attendant difference in its internal width along the x- and y-axes,
these two resonances will have (slightly) different resonant
frequencies. The exact resonance frequency values can be tuned by
(slightly) sliding either or both of the tuning elements 215, 217
into/out of the cavity 201; alternatively/supplementally, if the
wall 203 is pliable, the eccentricity of the cavity 201 can be
slightly altered, e.g. with the aid of (undepicted) actuators
(and/or by manual adjustment).
In a specific example, it is elected to have the resonant
frequencies of the cavity 201 at or close to a value of 3GHz. This
is not a limiting value: it is merely conveniently achievable, and
has an additional advantage of being the 40th harmonic of 75MHz.
This latter frequency is the pulse frequency of many commercially
available lasers, and such lasers can, if desired, be employed to
apply an external stimulus to a sample during examination in a CPM
(see above). It is thus relatively easy to phase-lock a 3GHz signal
from an oscillating power supply and a 75MHz output from a laser.
It is also relatively easy to tune the two resonant frequencies of
the cavity 201 to, for example, 3 GHz along the elliptical cavity's
major axis (x-axis) and 3.075GHz along its minor axis
(y-axis)--leading to a frequency difference of 75MHz, and thus
allowing excellent synchronization to light pulses produced with
such a laser. For the frequency value(s) in question, the lateral
dimensions of the cavity 201 will depend inter alia on the
dielectric medium present within the wall 203. For example: If the
dielectric is vacuum, one obtains a minor-axis length (a.sub.y) of
ca. 122 mm and a major-axis length (a.sub.x) of ca. 134 mm. The
power loss (P) for such a cavity is ca. 393 W, and its so-called
Quality factor (Q) has a value of ca. 11100, assuming a magnetic
field value of B.apprxeq.3 mT (milliTesla) in the cavity. In an
alternative case, the cavity 201 is largely filled with a ceramic
dielectric comprising ZrTiO.sub.4 doped with <20% SnTiO4. The
employed body of dielectric is circle-cylindrical in form, fills
the reference circle 205, and comprises a central 3 mm-wide shaft
to allow passage of the electron beam. The above-mentioned values
then become (approximately) a.sub.y.apprxeq.20 mm,
a.sub.x.apprxeq.22 mm, P.apprxeq.45 W and Q.apprxeq.2600. The
values of a.sub.x and a.sub.y scale according to .di-elect
cons..sub.r, where .di-elect cons..sub.r is the relative
permittivity of the employed dielectric (relative to vacuum), with
.di-elect cons..sub.r.apprxeq.37 in the current case.
The above example assumes a length of the cavity 201 (parallel to
the z-axis) of ca. 17 mm (though other values are, of course,
possible).
FIG. 1C schematically illustrates the field geometry of one of the
TM.sub.110 modes in the subject of FIGS. 1A and 1B, as follows:
Left drawing: The magnetic field (B) lies purely in a plane
parallel to the xy-plane. Close to the z-axis, it is oriented
substantially along the y-axis (front-to-back in FIG. 1C). Distal
from the z-axis, it demonstrates a whirlpool form. Right drawing:
The electric field (E) is zero in the x- and y-directions, and also
zero along the z-axis. To either side of the z-axis, it
demonstrates a clear dipole form, with field lines oriented up/down
parallel to the z-axis.
The other (simultaneously excited) TM.sub.110 mode in the cavity
201 will be basically the same, but will be laterally rotated (in
the xy-plane) through an angle of 90.degree., so that the magnetic
field (B) lines close to the z-axis point left-right instead of
front-back. In practice, the TM.sub.110 B-field excited in the
cavity will not be confined to a single plane: the situation
illustrated on the left of FIG. 1C will exist in any plane taken
normal to the z-axis (within the cavity).
The effect of the abovementioned simultaneous resonances will be to
superimpose an oscillatory x-deflection and an oscillatory
y-deflection on a charged-particle beam propagating along the
particle-optical axis 219. The resultant oscillation will cause
said beam to trace out a composite geometrical figure (a Lissajous
figure) on a (non-depicted) plane located downstream of the cavity
201 (beneath the aperture 221b in FIG. 1B). An example of such a
geometrical figure is illustrated in FIG. 1D. If a (non-depicted)
masking plate containing a small opening (such as a hole or slit)
is located in said downstream plane, and the plate's opening is
positioned so as to be intercepted by the geometrical figure, then,
as the beam traverses the opening, it will produce a
charged-particle pulse downstream of the masking plate.
In a specific configuration using the frequency values quoted
above, the inventors observed a 150 MHz repetition rate for the
figure depicted in FIG. 1D. When viewed along a given lateral
direction, each such repetition will involve an outward and a
homeward motion of the beam; if only one of these is selected, a
75MHz pulse frequency will be obtained. Using a masking plate
having an opening (hole) width of ca. 10 .mu.m and placed ca. 10 cm
downstream of the exit aperture 221b, one can create electron
pulses with a pulse length (duration) of ca. 100 fs for the
abovementioned magnetic field value of B.apprxeq.3 mT.
Embodiment 2
FIG. 2 renders a highly schematic longitudinal cross-sectional view
of a particular embodiment of a CPM in which the current invention
can be applied. In the present instance, the CPM is a TEM.
The depicted TEM comprises a vacuum housing 120 that is evacuated
via tube 121 connected to a vacuum pump 122. A charged-particle
source in the form of an electron gun 101 produces a beam of
electrons along a particle-optical axis (imaging axis) 100. The
electron source 101 can, for example, be a field emitter gun, a
Schottky emitter, or a thermionic electron emitter. The electrons
produced by the source 101 are accelerated to an adjustable energy
of typically 80-300 keV (although TEMs using electrons with an
adjustable energy of 50-500 keV, for example, are also known). The
accelerated electron beam then passes through a beam limiting
aperture/diaphragm 103 provided in a platinum sheet. To align the
electron beam properly to the aperture 103, the beam can be shifted
and tilted with the aid of deflectors 102, so that the central part
of the beam passes through the aperture 103 along axis 100.
Focusing of the beam is achieved using magnetic lenses 104 of a
condenser system, together with (part of the) final condenser lens
105. Deflectors (not depicted) are used to center the beam on a
region of interest on a sample, and/or to scan the beam over the
surface of the sample. In this schematic, functional depiction, the
deflectors 102 are shown relatively high up in the CPM, and final
condenser lens 105 is shown as being relatively small; however, the
skilled artisan will appreciate that deflectors 102 may be much
lower in the CPM (e.g. nested within the lens 105), and that item
105 may be much larger than depicted.
The sample to be examined is held by a sample holder 112 in such a
manner that it can be positioned in the object plane 111 of
projection system 106 (whose uppermost lens element is
conventionally referred to as an objective lens). The sample holder
112 may offer various positional/motional degrees of freedom (one
or more of translation(s), pitch, roll and yaw), and may also have
temperature control functionality (heating or cryogenic). It may be
a conventional type of sample holder for holding a static sample in
a containment plane; alternatively, the sample holder 112 can be of
a special type that accommodates a moving sample in a flow
plane/channel that can contain a stream of liquid water or other
solution, for example.
The sample is imaged by projection system (projection lens system,
projection column) 106 onto fluorescent screen 107, and can be
viewed through a window 108. The enlarged image formed on the
screen typically has a magnification in the range
10.sup.3x-10.sup.6x, and may show details as small as 0.1 nm or
less, for example. The fluorescent screen 107 is connected to a
hinge 109, and can be retracted/folded away such that the image
formed by the projection system 106 impinges upon image detector
151. It is noted that, in such an instance, the projection system
106 may need to be (slightly) re-focused so as to form the image on
the image detector 151 instead of on the fluorescent screen 107. It
is further noted that the projection system 106 may additionally
form intermediate images at intermediate image planes (not
depicted).
The image detector 151 may, for example, comprise a Charge-Coupled
Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS)
device, both of which can be used to detect impinging electrons. As
an alternative to electron detection, one can also use a CCD/CMOS
that detects light--such as the light emitted by a Yttrium
Aluminium Garnet (YAG) crystal (for example) that is bonded to the
CCD/CMOS, or connected thereto by optical fibers (for example). In
such an indirect detector, the YAG crystal emits a number of
photons when an electron hits the crystal, and a portion of these
photons is detected by the CCD/CMOS; in direct detectors, electrons
impinge on the semiconductor chip of the CCD/CMOS and generate
electron/hole pairs, thereby forming the charge to be detected by
the CCD/CMOS. The detector 151 is connected to a processing
apparatus (controller) and display unit [not depicted].
The image formed on the fluorescent screen 107 and on the image
detector 151 is generally aberrated due (for example) to
imperfections produced in the projection system 106. To correct
such aberrations, various multipoles can be deployed in/near the
projection system 106. Such multipoles are not depicted in FIG. 2,
so as to avoid cluttering the drawing, but the skilled artisan will
be familiar with their design, positioning and implementation.
Where the imaging beam impinges on the sample 111, "stimulated
radiation" is generated in the form of secondary electrons, visible
(fluorescence) light, X-rays, etc. Detection and analysis of this
radiation can provide useful information about the sample 111. To
achieve such detection, FIG. 2 shows a supplementary detector 130,
which is connected to a voltage source 132. As here depicted, the
detector 130 is positioned at the side of the sample plane 111
proximal the gun 101; however, this is a matter of design choice,
and a detector 130 may alternatively be positioned at the side of
the sample plane 111 distal the gun 101, for example. The
"detector" alluded to in the appended claims may be either or both
of detectors 151 or 130, or another (undepicted) detector.
In addition to the detectors 151/130, the depicted apparatus may
also be equipped with EELS or EFTEM functionality, for example. In
this context: For EELS: Deflectors 152 can be used to deflect
transmitted electrons (traversing the sample) in a direction away
from the optical axis 100 and toward an off-axis EELS detector,
which is not shown in FIG. 2. For EFTEM: Use can be made of an
energy "filter", whose purpose is to select which energy range of
electrons will be admitted to the detector 151 at any given time.
Such filter functionality can be fulfilled by the deflection coils
152, which will "pass" certain electron energies while deflecting
others aside.
It should be noted that FIG. 2 only shows a schematic rendition of
a (simplified) TEM, and that, in reality, a TEM will generally
comprise many more deflectors, apertures, etc.
In the context of the present invention, it is desirable to be able
to pulse/chop the electron beam before it impinges on the sample
being investigated. To this end, a (non-depicted) beam pulsing
device according to the present invention is disposed about the
particle-optical axis 100 at some point between the source 101 and
the sample holder 112, preferably at a crossover point, e.g. a
focal point of the penultimate condenser lens 104. This pulsing
device will comprise a unitary resonant cavity as set forth above,
e.g. similar to that in FIGS. 1A/1B and Embodiment 1.
Embodiment 3
FIG. 3 renders a schematic longitudinal cross-sectional view of
another embodiment of a CPM in which the current invention can be
applied. In the present instance, the CPM is a SEM.
In FIG. 3, a SEM 400 is equipped with an electron source 412 and a
SEM column (particle-optical column) 402. This SEM column 402 uses
electromagnetic lenses 414, 416 to focus electrons onto a sample
410, and also employs a deflection unit 418, ultimately producing
an electron beam (imaging beam) 404. The SEM column 402 is mounted
onto a vacuum chamber 406 that comprises a sample stage 408 for
holding a sample 410 and that is evacuated with the aid of vacuum
pumps (not depicted). The sample stage 408, or at least the sample
410, may be set to an electrical potential with respect to ground,
using voltage source 422.
The apparatus is further equipped with a detector 420, for
detecting secondary electrons that emanate from the sample 410 as a
result of its irradiation by the imaging beam 404. In addition to
the detector 420, this particular set-up (optionally) comprises a
detector 430, which here takes the form of a plate provided with a
central aperture 432 through which imaging beam 404 can pass. The
apparatus further comprises a controller 424 for controlling inter
alia the deflection unit 418, the lenses 414, 416, the detectors
420 and 430, and displaying obtained information on a display unit
426.
As a result of scanning the imaging beam 404 over the sample 410,
output radiation, such as secondary electrons and backscattered
electrons, emanates from the sample 410. In the depicted set-up,
secondary electrons are captured and registered by the detector
420, whereas backscattered electrons are detected by detector 430.
As the emanated output radiation is position-sensitive (due to said
scanning motion), the obtained (detected/sensed) information is
also position-dependent. The signals from the detectors 420 and
430, either severally or jointly, are processed by the controller
424 and displayed. Such processing may include combining,
integrating, subtracting, false coloring, edge enhancing, and other
processing known to the skilled artisan. In addition, automated
recognition processes, such as used in particle analysis, for
example, may be included in such processing.
In an alternative arrangement, voltage source 422 may be used to
apply an electrical potential to the sample 410 with respect to the
particle-optical column 402, whence secondary electrons will be
accelerated towards the detector 430 with sufficient energy to be
detected by it; in such a scenario, detector 420 can be made
redundant. Alternatively, by substituting one or more of the
detectors 420 for the detector 430, these detectors 420 can assume
the role of detecting backscattered electrons, in which case the
use of a dedicated detector 430 can be obviated. In light of such
variants, the "detector" alluded to in the appended claims may be
either or both of detectors 420 or 430, or another (undepicted)
detector.
If desired, one can realize a controlled environment (other than
vacuum) at the sample 410. For example, one can create a pressure
of several mbar, as used in a so-called Environmental SEM (ESEM),
and/or one can deliberately admit gases--such as etching or
precursor gasses--to the vicinity of the sample 410. It should be
noted that similar considerations apply to the case of a TEM, e.g.
as set forth in Embodiment 2 above, whereby a so-called ETEM
(Environmental TEM) can be realized, if desired.
Once again, in the context of the present invention, it is
desirable to be able to pulse/chop the electron beam 404 before it
impinges on the sample 410 being investigated. To this end, a
(non-depicted) beam pulsing device according to the present
invention is disposed about the particle-optical axis of the SEM
400 at some point between the source 412 and the sample holder 408,
preferably at a crossover point, e.g. a focal point of the lens
414. This pulsing device will comprise a unitary resonant cavity as
set forth above, e.g. similar to that in FIGS. 1A/1B and Embodiment
1.
Embodiment 4
FIG. 4 renders a schematic longitudinal cross-sectional view of yet
another embodiment of a CPM in which the current invention can be
applied. In the present instance, the CPM is a FIB microscope.
FIG. 4 shows a FIB tool 500, which comprises a vacuum chamber 502,
an ion source 512 for producing a beam of ions along a
particle-optical axis 514, and a FIB column (particle-optical
column) 510. The FIB column includes electromagnetic (e.g.
electrostatic) lenses 516a and 516b, and a deflector 518, and it
serves to produce a focused ion beam (imaging beam) 508.
A workpiece (sample) 504 is placed on a workpiece holder (sample
holder) 506. The workpiece holder 506 is embodied to be able to
position the workpiece 504 with respect to the focused ion beam 508
produced by the FIB column 502.
The FIB apparatus 500 is further equipped with a Gas Injection
System (GIS) 520. The GIS 520 comprises a capillary 522 though
which a gas may be directed to the workpiece 504, and a reservoir
524 containing the gas (or a precursor substance used to produce
the gas). A valve 526 can regulate the amount of gas directed to
the workpiece 504. Such a gas may be used in depositing a
(protective) layer on the workpiece 504, or to enhance a milling
operation performed on the workpiece 504, for example. If desired,
multiple GIS devices 520 may be employed, so as to supply multiple
gases according to choice/requirement.
The FIB tool 500 is further equipped with a detector 530, which, as
here embodied, is used to detect secondary radiation emanating from
the sample 504 as a result of its irradiation by the ion beam 508.
The signal from the detector 530 is fed to a controller 532. This
controller 532 is equipped with a computer memory for storing the
data derived from this signal. The controller 532 also controls
other parts of the FIB, such as the lenses 516a/b, the deflector
518, the workpiece holder 506, the flow of the GIS 520 and the
vacuum pumps (not depicted) serving to evacuate the chamber 502. In
any case, the controller 532 is embodied to accurately position the
ion beam 508 on the workpiece 504; if desired, the controller 532
may form an image/spectrum of detected/processed data on monitor
524.
Again, in the context of the present invention, it is desirable to
be able to pulse/chop the ion beam before it impinges on the sample
504 being investigated. To this end, a (non-depicted) beam pulsing
device according to the present invention is disposed about the
particle-optical axis 514 at some point between the source 512 and
the sample holder 506, preferably at a crossover point, e.g. a
focal point of the lens 516a. This pulsing device will comprise a
unitary resonant cavity as set forth above, e.g. similar to that in
FIGS. 1A/1B and Embodiment 1.
* * * * *